<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
</body>
</html>
<script>
    var a = Number(prompt('输入一个数'))
    if(a>=80 && a<=100){
        alert('优秀')
    }else if(a>=70 && a<80){
        alert('良好')
    }else if(a>=60 && a<70){
        alert('合格')
    }else if(a<60 && a>=0){
        alert('不及格')
    }else{
        alert('没有这个数')
    }
</script>